Compressor flow control means



Feb. 19, 1957 v. MOORE 2781634 COMPRESSOR FLOW CONTROL MEANS Filed Nov. 1, 1950 2 Sheets-Sheet 1 ATTDRNEY Feb. 19, 1957 v. MOORE COMPRESSOR FLOW CQNTROL MEANS 2 Sheets-Sheet 2 Filed Nov. l, 1950 INVENTOR. vmczwr MI.'.ICIRE ATTCIRNEY Unitecl States Patent Oflice 2,781,634 Patented Feb. 19, 1957 2,781,634 COMPRESSOR FLOW CONTROL MEANS Application November 1, 1950, Serial N0. 193,376 1 Claim. (Cl. 6035.6)

This invention relates to compressorg and to gas turbine ower plants and is particularly directed to means for improving the erformance of a compressor of the axial flow and/er the centrifugal types and the erformance of a gas turbine power plant having such a compressor.

At a given speed of rotation, a compressor of the axial flow or centrifugal type operates satisfactorly over only a limited range of air flow therethrough. This is particularly true of multi-stage axial flow compressors. When the flow of air or other gaseous fluid through the compressorfalls below this range, at least the initial stage or stages of the blades of a multi-stage axial flow compressor stall and a -compressor surge cndition may develop. The number of stages of the compressor blades which must stall before the cornpressor surges will vary with the compressor design. In the case 0f an air compressor of a gas turbine power plant, such as for aircraft, the compressor is required to operate over a wide range of air flow therethrough as well as over a wide speed ra'nge. An objeot 0f the present invention comprises the provision of automatic means for preventing the occurrence 0f compressor surge and/er Stall conditions notwithstanding compressor operation over a Wide speed and air flow range.

In accordance with the present invention air is bled oft from a compressor at some point downstream of the compressor inlet and the rate of this air bleed is automatically controlled so as to prevent stalling of the compressor. For this latter purpose said rate of air bleed is automatically controlled by means responsive to changes in the angle of attack of the compressor air at a particular stage of compressor blades upstrearn of the'stage at which said air is bled elf, the rate of air bleed being varied so that said angle of attack is maintained at' a desired value.

A gas turbine power plant for aircraft conventionally comprises a combustion chamber assernbly, an air'compressor assetnbly for supplying s-aid combustion chamber With compressed air f01 combustion and for -cooling the combustion gases, and a turbine assembly driven by the gases frorn the combustion chamber. In addition said turbine assernbly is drivably connected to the compressor assembly and the turbine exhaust gases discharge rearwardly through a nozzle to provide the power plant With forward propulsive thrust. In the case of a gas turbine power plant incorporating said invention, the air bled 0fi? from the c'ompressor is discharged rearwardly through an auxiliary nozzle to add to the propulsive thrust of the power plant. The addition of said air bleed and auxiliary nozzle to a gas turbine power plant not only miniximizes the occunrence of surge and/or stall conditions 'in the compressor but also improves the performance of the gas turbine ower plant by increasing the pdwer plant thrust for a given rate of fuel consumption.

Other objects of the invention will become apparent upon reading the anhexed detailed description in conneetion With the drawing in which:

Figure 1 is a schematic seotional view of an airoraft gas. turbine: power plant iz'mbodying the invention;

Figure 2 is an enlarged sectional view of a portion of Figure 1 Figure 3 is a view taken along l1'ne 22 of Figure 1 and illustrating various velocity vector thereon;

Figure 4 is a view similar to Figure 3 but illustrating the angle of attack responsive means disposed ahead of a stage of stator blades instead of, as in Figure-3, ahead of a stage of rotor blades;

Figure 5 is a diagrammatic sectional view of a modified form of angle of attack responsive means;

Figure 6 is a secti0nal view taken along line 66 of Figure 5; and

Figure 7 is a schematic partial view illustrating the invention applied to a centrifugal type compressor.

Referring first to Figures 1 and 2 of the drawing, a gas turbine power plant 10for propulsion of aircra ft comprises a housing 12 in which two multi-stage axial flow compressors 14 ahd 16 are disposed in series. The upstream axial flow cornpressor c0mprises a plurality of stator blades 18 extending radially inwardly from the housing 12 into the annular air flow path and alternating wit'h a ph1rality of stages of roter blades 20 extending radially outwardly from the compressor roter into said path. The downstream or second axial flow compressor 16 similarly comp-rises a plurality of stages of stator blades 22 alternating with a plurality of stages of rotor blades 24. The compressor 14 receives air through an annular air intake duct 26 and said compressor discharges compressed air into an annular duct 28 from which said air enters the second compressor 16. The second compressor 16 further compresses said air and supplies it to a combustion chamber 30 to which fuel is supplied through nozzles 32. The discharge and of the combustion chamber is provided with vanes or nozzles 34 which direct the gases discharging -therefrom against the blades 36 of the turbine rotor 38 for driving said rotor. The exhaust gases from the turbine discharge rearwardly from the power plant into the surrounding atmosphere through a nozzle 40 to provide the power plant with forward prc pulsive thrust. The tail cone 41 of the power plant may be axially movable for varying the area of the exhaust nozzle opening 40 as is quite common, for example see Patent Numbers 2,520967 and 2529,973. The turbine roter 38 is drivably connected to the cornpressors by the shaft 42. The turbine 38 may also be connected to a conventional 2iircraft 'propellen (no t sh=own) for providing at least a portion of the ower plant thrust as is well known, for example see Patent Number 2529973. The

compressors 14 and 16 may be driven by the turbine roter 38 at the same speed or change gearing (not shown) may be interposed between said compressors. This latter feature is also well known, for example see Fig. 14 of Patent Number 2,396,911. In addition the power plant may include two serially disposed turbines each connected to one of the compressors (for example as illustrated in the copending R. W. Cola application Serial N0. 25,942 filed May 8, 1948, now abandoned) and one of said turbines may also be drivably connected to said propeller for providing at least a portion of the power plant thrust. The gas turbine power plant structure so far described is conventional.

If at a gi ven compressor rotative speed, the volume air flow rate"through the compressor assembly is progressiveljr decreased a p0int Will be reaohed at which the angle -of attack of the air on the first stage of cornpressor blades becomes so large that said stage stalls. Upon still further reduction in said volume flow rate the second stage of the compressor blades Will stall, etc. and the conipressor assembly generally will go into a surge comdition before all of its stages reach a stalling condition. It is known that bleeding air from the compressor a-ssembly increases the volume flow rate through the comnressg;

and therefore eliminates or at least mim'rnizes said sorge. Since the first er upstream stages of the compressor blades stall first, the air need not be bled frorn the discharge end of the compressor assembly but can be bled fron1 the compressor assembly at some intermediate stage. Figure 1 illustrates the eompressor assembly as comprising two compressors 14 and 16 in series so that the most comvcnient place for bleeding air from said assembly is be- Lween the two cornpressors 14 and 16. For this purpose an annulus 44 communicates with the assage 28 between the two compressors 14 and 16 and a duct 46 extencls rearwardly from the annulus 44 and terminates in a rearwardly directed discharge noz2le 48. It will become apparent that the second cornpressor 16 is not essential to applicants invention and can be dispensed With, in which case the bleed duct 46 could still communicate with the compressor assemloly 14 at the discharge end of said assembly or said duct could communicate with the assembly 14 at an interrnecliate stage.

The rate cf air bleed through the duct 46 is controlled by a valve 50 which is axially rnovable to vary the area of the nozzle 48, a piston and cylinder motor 52 being provicled for moving said valve. In accordance with the invention, the valve 50 is automatically controlled 50 as 10 rnaintain a constant desired angle of attack n a particular stage of the compressor blades. For this purpose, a shaft 56 is journaled in bean'ngs 58 carried by the roter of the compressor 14, said shaft extending transversely across said roter with one end of said shaft projecting into the air flow path. A vane 60 is fixedly secured to said shaft end adjacent to the leading edge of said vane whereby said vane automatically aligns itself with the direction of the local air flow relative to -the rotor of the compressor 14. As illustrated, the vane 60 is disposed between the first stage stator blades 18 and the first stage rot0r blades 20 of the compressor 14, the axially spacing of said first stage stator and roter blades being exaggerated in Figure 1 to permit clear illustration cf the vane 60. Because the vane 60 is hingedly nrounted adjacent to its leading edge, the centrifugal force a-cting on said vane tends to rotate said vane about it hinge axis. This eflect of centrifugal force on the vane 60 is eliminated by suitable counterweight means, schematically indicated at 61, whereby the vane 60 responcls only to changes in the direction cf and aligns itself with the local air flow.

In order to connect the vane 60 \Vith a servo-mechanism 62, the shaft 56 is provided with a gear 63 meshing with a rack 64 formed on a rod 66 also rotatable With the rotor 0f the cornpressor 14. The rod 66 has a swivel connection a1; 68 with a spool-shaped servo-valve 70 slidable in a cylincler 72 whereby the axial position of the valve 70 depends on the angular position of the vane 60 about its hinge axis. The head ends 74 and 76 of the servo-valve 70 cooperate witl1 radial holes er orts 78 and 80 respectively in the cylinder 72 and said ports respectively communicate With elongate grooves 82 and 84, extending along a bore 85 within the fixed housing of the servomechanism 62 and Within which the cylinder 72 is slidably mounted. Conduits 86 and 88 are connected at one end with the gr0oves 82 and 84 respectively and at their other ends with the opposite ends of the piston-cylinder motor 52. A suitable Fluid (such as oil) under pressure is supplied to the -servo-mechanism through a passage 90, said passage terminating in an elongate groove 92 extending along the bore 85 of the serv0-mechanism housing and a radial hole or port 94 in the cylinder 72 supplies said lluid from the groove 92 to the annulus 96 between the head ends 74 and 76 of the servo-valve 70.

Springs 98 and 100 are provided between the servovalve 70 and the head ends of the cylinder 72 to bias the servo-valve 70 to a central position therein. In addition the cylinder 72 is provided with drain holes 102 and 104 at its ends, said holes communicating with elongate grooves 106 and 108 and passages 110 and 112 respectively in the housing of the servo-mechanism 62. The cylinder 72 also has an axially extending. portion 114 (througl1 which the rod 60 extends) said porti0n having an annular groove 116 forrned thereon within which one end of a lever 118 is received so that the axial osition of the cylinder 72 within the fixed housing of the servo mechanisrn 62 is controlled by said lever.

Referring to Figure 3, air leaves the stator blades 18 in a direction tangent to the trailing edges of said blades. In Figure 3, the rotor blades are assumed to move in the direction of the arrow 119. If the vector Vs represents the velocity cf flow 0f the air relative to the stator blades as said air leaves said stator blades and if the vector VI) represents the velocity of the rotor blades, then the vector VI, representing the velocity of the air relative to said roter blades is obtained by cornbining the vectors Vs und Vb and x designates the angle of attack of the air relative to said roter blades. For a given value of the vector V b, it is apparent that the angle of attack x of the air with respect to the first stage roter blades will incrcase and decrease with decrease and increase respectively in the vect0r Vs and -for a given value of the vector Vs the angle of attack x will increase and decrease respectivcly with increase and decrease in the velocity V11 of the roter blades. 'Il1is obviously is true regardless of the stage of roter blades 20 ahead of which the vane 60 is disposcd.

With the structure described, if the angle of altack x of the air on the first stage roter blades 20 incrcascs, the vane 60 will swing clockwise (as viewed in Figure 3) about the axis of the shaft 56 to align itself with the ncw direction of the air flow relative to said roter blacles 20. This rnovement of the vane 60 m'oves the rod 66 and serv0-valve 70 a proportional distance to the righl. (as viewed in Figures 1 and 2) whereupon the passage 86 is vented at the ort 78 to the left band end of the cylinder 72 and fluid uncler pressure is supplied from the annulus 96 through the port to the passageway 88 for moving the nozzle valve 50 in a direction to elfect an opening of the nozzle 43. The nozzle valve 50 moves in an open ing direction until the increase in the bleed air flow therc through increases the vector Vs to an extent sufi1cient to restore the angle of attack x to its original value whereupon the servo-valve 70 again similarly eovers both ports 78 and 80. In this way the servo-rnechanism 62 is aute matically operative to position the nozzle valve 50 for varying the rate of air bleed from the compressor assembly so that the angle 0f attack x 01 the air on the first stage rotor blades is maintained constant. The magnitude of said angle of attack can be set by axially adjusling the osition cf the cylinder 72 by means 0f the lever 118 so that the mechanism can be set to maintain the angle of attack x below the stalling angle of said first stage rotor blades. F0r example, movement of the cylinder 72 to the left (Figure 2) vents the port 78 und causes fluid under pressure to be supplied to the passage 88 through the port 80 to effect an opening adjustment of the nozzle valve 50 thereby increasing the vector Vs and decreasing the angle of attack x whereupon the vane 60 swings counterclockwise (Figure 3) to move the servo-valve 70 to the left until said valve again similarly Covers both ports 78 and 80. Accordingly the angle of attack x maintained by the servo-mechanism 54 can be increasecl or decreased by moving the cylinder 72 to the right or left respectively (as viewed in Figure 2).

For any given axial flow compressor, it can readily be established that the angle of attack x on any stage 01 the compressor rotor blades varies inversely with a quantity Q/N in which Q is the volume air flow rate through said compressor and N is the rotative speed (revolutions per minute) of said compressor. Aceordin=gly by maintaining a constant angle of attack x the quantity Q/N is also maintained constant. The quantity Q/N is a basic factor affecting the performance of both axial flow and centrifugal compressors.

With the arrangement described, the vane 60 is rotatable With the compressor rotor blades" and saidvane automatically aligns itself With the direction of air flow relative t the roter blades disposed immediately downstream from the vane 60. That is the vane 60 automatically aligns itself With the vector Vr representing the velocity of air flow relative to said roter blades. Actually however the angle of attack responsive means could be mounted on the compressor stator structure ahead of a stage of stator blades since an increase in the angle of attack on a particular stage of rotor blades is accompanied by a corresponding increase in the angle of attack on the succeeding stage of stator blades. That this is so can be seen in Figure 4.

Figure 4 illustrates two stages of stator blades 18 With an interposed stage of rotor blades 20. As in Figure 3, the vector Vs represents the exit velocity of the air relative to and from a stage of stator blades, the vector Vb represents the velocity of the rotor blades and the vector Vr represents the velocity of the air relative to said roter blades whereupon the angle x designates the angle of attack of the air on said roter blades. The exit velocity of the air relative to and from the rotor blades is tangent to the trailing edge of said blades as indicated by the vector Vr'. The velocity Vs of the air relative to the succeeding stage of stator blades is detennined by the combination of Vr With Vb whereby the angle y represents the angle of. attack of the air on said stage of stator blades. I t is apparent from Figure 4 that any increase in the angle of attack x, because for example of an increase in Vb or a decrease in Vs, is accompanied by a corresponding increase in the angle of attack y.

Not only may the angle of attack responsive means be disposed on the compressor stator ahead of a stage of stator blades instead Of, as illustrated in Figure l, on the roter ahead of a stage of rotor blades but in e-ither case other fornrs of angle of attack responsive means may be provided in place of the vane 60 and the associated servo-mechanism. For example, Figures 4 and illustrate a modified form of angle of attack responsive means 120 which is mounted on the stator ahead of a stage of stator blades 18.

As illustrated in Figures 4 and 5, the angle of attack responsive means 130 comprises a pair of similar Pitot tubes 122 and 124 having their open ends inclined to each other and extending through the wall of a tubular casing 126 within which said Pitot tubes are disposed. The Pitot tube casing 126 is supported on the compressor stator Wall 128 (from which the stator blades 18 project radially inwardly) ahead of a stage of stator blades 18, said casing 126 being oriented so that the bisector 130 of the angle between the open ends of the Pitot tubes 122 and 124 is parallel to the vector Vs. With the Pitot tubes so oriented, relative to the direction of air flow, the pressures in said tubes are equal. Upon a change in said direction of the air flow the fluid pressure in one or the other.of said Pitot tubes, depending on the direction of said change, will decrease relative to the pressure in the other tube. Figure 5 illustrate:s an arrangement whereby any pressure diiference between the Pitot tubes 122 and 124 is used to vary the position of the nozzle bleed valve 50 so as -to restore parallelism between the vector Vs and the Pitot tube bisector 130 whereby a constant angle of attack y is maintained.

In Figure 5, the Pitot tube casing 126 is supported by and projects through the stator wall 128, said casing having an annular flange 132 seated on an external surface of said stator Wall. A plate 134 is secured to the stator Wall by screws 136, said plate having a portion extending over the flange 132 to hold said casing 126 in position while permitting rotative adjustment of said casing about its axis. The Pitot tubes 122 and 124 extend from the casing 126 through the plate 134 to pressure fittings 140 and 142 respectively which are connected to a pair of opposed chambers 144 and 146 by passageways 148 and 150 respectively. The chambers 144 and 146 are separated by aflexible diaphra gm 152, said diaphragm being con nected to a spool-type servo-valve 158 by a rod 160.

The valve 158 is slidable in a bore 161 having vented ends, the head portions 162 and 164 of said valve 158 cooperating With passageways 166 and 168 respectively. A fluid under pressure is supplied to the annular space 170 between the head portions of the valve 158 through a passage 172. The passages 166 and 168 correspondto the passages 86 and 88 respectively cf Figure 1 so that the fluid pressure in the assage 166 tends to close the nozzle bleed valve 50 and the fluid pressure in the passage 168 tends to open said valve.

Centering springs 174 and 176 are provided for urging the valve 158 to its central or neutral pos-ition (illustrated in the drawing) in which the head ends 162 and 168 of said valve similarly cover their respective passages 166 and 168. The Pitot tube casing 126 has an arm 178 projecting from its flange 132 and the Pitot tubes 122 and 124 are sufliciently flexible to pernrit said casing to be rotatably adjusted about its axis by the arm 178 in order to change or set the direction of the bisector 130.

With the arrangement of Figures 4 and 5, if the bisector is aligned With the direction of the air flow relative to and entering the adjacent stage of stator blades 18, then the fluid pressures in the Pitot tubes 122 and 124 will be equal so that the servo-valVe 158 Will be centered in the pos'ition illustrated. If however the direction of said air flow should change for exarnple to decrease the angle of attack y, then the pressure in the Pitot tube 122 will increase relative to the pressure in the tube 124 whereupon the diaphragm 152 Will be subjected to a pressure diiferential urging the servo-valve 158 to the left (Figure 4). Movernent of the servo-valve 158 to the left Will ad- 1nit fluid under pressure to the passage 166 to efiect a closing adjustment of the nozzle bleed valve 50 thereby increasing said angle of attack back to the value at which the bisector 130 is again parallel to the vector Vs and the fluid pressur'esin the Pitot tubes are again equal. Similarly anincrease in the angle of attack y results in a closin=g adjustment of nozzle bleed valve 50 until the angle of attac k y is decreased to the point at which parallelisrn of the vector Vs and the bisector 130 and equality of the Pitot tube fluid pressures is again established. Accordingly the arrangement of Figures 4 and 5 operates to vary the compressor air bleed so as to rnaintain the vector Vs parallel :to the Pitot tube b-isector 130 whereby the magnitude of the angle of attack y maintained by the system can be changed by changing the direction of said bisector. As already described the direction of said bisector 130 can be set or changed by rotating the Pitot tube casing 126 by means of the arm 178.

Although the arrangement of Figures 4 and 5 responds to changes in the fluid pressures in the Pilot tube 122 and 124, said arrangement is independent of changes in the density and/er velocity of the air. This is so because the rate of compressor air bleed is automatically controlled t0 rnaintain zero pressure differential between said Pitot tubes and this condition cf the Pitot tubes depends only on the direction of the air flow being parallel to the bisector 130 and is independent of the velocity and/er density of said air.

The invention so far has been illustrated and described in connection With an axial flow compressor. As will appear however the invention is not so limited but is also applicable to a centrifugal type of fluid compressor since in both types, compressor performance depends on the angle of attack With which the compressor fluid approaches each stage of the compressor blades, said angle varying With the previously described factor Q/N. In

the case of a positive-displacement type of compressor, however, the factor Q/N is constant (neglecting leakage losses) so that said factor is not one of the variables aflecting the performance of positive-displacement type compressor-s. In the appendant claims, the expression nun-positive displacement type compressor is intended 10 include compressors of the axial flow and centrifugal types as well as cornbinations of axial flow and centrifugal compressors.

Figure 7 illustrates an application of the invention to a centrifugal type compressor assembly having a first stage centrifugal cornpressor 200 and having a second stage centrifugal compressor 202. The first stage Centrifugal compressor 200 comprises a rotor 204 having a plurality of circumferentially-spaced blades 206 projecting therefrom into the annular air flow path and a plurality of circumferentially-spaced fixed diffuser blades 208 a1e disposed in said path downstrearn cf the rotor blades 206. Similarly the second stage compressor 202 comprises a rotor 210 having a plurality of circurnferentially-spaced blades 212 projecting therefrom intofhe annular air flow path and a plurality of circumferentiallyspacecl diffuser blades 214 are disposed in said path downstrearn cf tl1e roter blades 212. The fi1'st stage ccntrifugal cornpressor 200 receives air frorn an annular air intake duct 216 and compresses said air into an nnnular passage 218 from which the second stage com pressor 202 further compresses said air into an annular discharge passage 220. As in Figure 1 the compressor assexnbly of Figure 7 may form part of a gas turbine power plant in which case the compressor assernbly supplies air through the passage 220 to a combustion chan1- her for supplying a gas turbine with motive fluid, said tureine being drivably connected to a shaft 222 for driving the compressor assembly. A duct 224 communicates with the compressor assembly betvveen 1ts first and second stages for bleeding air therefrom and a nozzle valve 226, at the rearwardly directed discharge end of the duct 224, controls the rate of air bleed through said duct.

As in Figure l, the bleed nozzle valve 226 of Figure 7 is controlled to maintain a substantially constant angle of attack on a stage of compressor blades, for exarnple blacles 206, upstream of the point at which air is bled off from the compressor assembly. For this purpose any n1eans responsive to the angle of attack of the air on the blades 206 rnay be used to control the nozzle valve 226 so 215 10 increase o1 decrease the rate of said air bleed in response to an increase or decrease respectively in said angle of attack. In Figure 7 a vane 228 is secured to a shaft 230 in the air stream upstream of the roter blades 206 f01' alignment Wil'h the direction of said air-stream relative to said roter blades. The vane 228 may control the nozzle valve 226 by mecllanism similar to that illustrated in Figures 1 and 2 in which the vane 60 controls ll1e nozle valve 50.

Obviously the angle of attack responsive rneans cf Figurcs 5 and 6 n1ay be used in Figure 7 in place of the vane 228 and its associated mechanism. Furthermorc, in Fig. 7 as in Fig. 1, the air bleed need not be talcen from the discharge and of the compressor and instead, as illustrated in Fig. 7, the air preferably is bled ofi at an intermediate stage 015 the compressor.

While 1 have described my invention in detail in its present preferred embodirnent, it will be obvious to those skilled in the art, after understanding my invention, that various changes and modifications may be made therein without departing fron1 the spirit or scope thereof. I

aim in the appended claim to cover all such modifications.

1 claim as my invention:

A gas turbine power plant for aircraft or other vehicles; said power plant comprising a cornbustion chamber; an axial flow air compressor assernbly having a plurality of stages of compressor blades for supplying said combustion chamber With air for combustion with fuel in said chamber; a turbine assembly drivably connected to said compressor assembly and arranged to be driven by gases from said combustion chamber; a first duct having a rearwardly directed nozzle through which the exhaust gases discharge from the turbine assernbly; a second duct communieating With said compressor assembly at an intermcdiate stage of said compressor assembly for bleeding off air from said compressor assembly, said second duct having a rcarwardly dirccted nozzle at its discl1arge end through which said bleed air discharges into the surrounding atmosphere to add t0 the power plant thrust; rneans for adjusting said second duct nozzle Tor varying its effective discharge arca thereby controlling the rate of said air bleed so that said air bleecl is a maximum when said second duct nozzle is open to its maximum extent and when said second duct nozzle is closcd all the comprcsso-r ai1 contributing substantially to thc pow 3rplant thrust is supplied to said combustion chamber; and coutrol mechanism f0r causing operation of said nozzle varying means to maintain a prccleterrnined angle cf attack of the compressor air on a stage of compressor blades upstream of said intermediate stage; said control mechanisrn including means responsive to changes in the direction of the fluid flow over said upstream stage of compressof blades, mcans adjustable for setting thc magnitude of said predetermined angle of attack, means operatively connected to said responsive means and movable in response I0 deviation in said angle from the set value in a direciion and to an extent dependent on the direction and magnitucle of said cleviation, and means operatively connecting said rnovable means to said nozzle varying means for adjusting said second duct nozzle upon deviation of said angle of attack from the set value so as to restore said angle of attack said Set value by increasing o1 decreasing said air bleed to an extent depenclent 0n the magnitude of the increase o1 decrease respeetively in said angle of attack from said set value.

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